The invention relates to motion sensors. More specifically, the invention relates to an optical encoder including a photosensor array.
An inkjet printer typically includes a rotary optical encoder and a linear optical encoder. A patterned encoder wheel of a typical rotary encoder is attached concentrically to a shaft or roller forming a part of a drive mechanism that moves a print medium through the printer. The patterned wheel is typically a thin disc having multiple encoder marks or slots. The marks or slots create a repetitive pattern of radially-extending reflective and non-reflective regions, or opaque and transparent regions, around the disc. When the shaft or roller is rotated to move a sheet of paper or other print medium through the printer, the encoder wheel of the rotary encoder is also rotated.
An encoder assembly of the typical rotary encoder includes one or two photosensor elements, an illuminator and perhaps some filters and lenses. The illuminator is used to illuminate an area of the patterned wheel during the operation of the printer. When the patterned wheel is rotated, the marks or slots pass through the illuminated area. The marks or slots modulate light detected by the photosensor elements at a rate that is proportional to the rate of angular rotation of the patterned wheel. Such modulated light causes each photosensor element to output a train of pulses at a frequency that is proportional to the rate of angular rotation. This feedback information is supplied to a closed loop controller, which controls the feed rate of the sheet.
When two photosensor elements are used, they are typically placed relative to one another such that their output pulses have a quadrature relationship. Such a relationship allows rotation, direction and angular displacements of one-quarter the period of the marks or slots to be detected.
The linear optical encoder is used for detecting and measuring movement of a print head across the print medium. A typical linear encoder includes a patterned encoder strip having alternating marks or slots. The marks or slots create a repetitive pattern of reflective and non-reflective regions, or opaque and transparent regions, along the strip. The typical linear optical encoder further includes an encoder assembly having an illuminator and at least two photosensor elements arranged in a quadrature relationship. The encoder assembly is secured to the print head and is thereby movable relative to the patterned strip. When the print head is moved relative to the patterned strip, the marks or slots modulate light detected by each photosensor elements at a rate that is proportional to the rate of linear movement of the print head. The photosensor elements, in turn, output a train of pulses at a frequency that is proportional to the linear movement of the print head. Feedback information from the encoder assembly is supplied to the closed loop controller, which controls the linear rate or position of the print head.
Patterned targets such as patterned encoder wheels and patterned encoder strips are designed in accordance with certain specifications. These specifications include pitch of the marks or slots and mark/space ratio. Encoder assembly specifications include photosensor element location, how accurately the patterned target is aligned (and centered for rotary encoders), high/low signal ratio, defect density, etc. If any one of these specifications is violated, encoder accuracy might be unacceptable and the optical encoder might not function correctly in a printer.
Using patterned targets increases the cost and difficulty of manufacturing and aligning the encoders. On the other hand, if patterned targets were not required, the encoders would become more robust. Additionally, manufacture and assembly of the optical encoders would be simplified and a changing target could be tolerated. The cost of labor and material would also be reduced. Therefore, eliminating the need for patterned targets would reduce the cost of the optical encoders.
Reducing the cost of the optical encoders, in turn, would reduce the cost of the printers. Mass market products such as printers are extremely cost-sensitive. Reducing the cost by a few pennies might seem trivial, but it can have a significant impact on the profitability of a printer manufacturer, especially a printer manufacturer selling millions of printers per year.
It would be desirable to detect relative motion of a target in an optical encoder without the use of a patterned target.